ES2305049T3 - PROCEDURE FOR THE MANUFACTURE OF STEEL OBJECTS FOR TOOLS THROUGH DUST METALURGY. - Google Patents

Abstract

Production of thick deformed or non-deformed objects made from tool steel comprises pouring a melt into a metallurgical vessel; conditioning to improve the degree of purity; adjusting the temperature to a value above the formation temperature of primary precipitation in the alloy; forming a powder having an average grain size of 50-70 mu m from the melt at constant temperature by injection nitrogen; disintegrating in the nitrogen stream; grading the powder; collecting, mixing and pouring into a container and compacting by hot isostatic pressing. The parameters of the pressing cycle are adjusted so that the temperature and pressure in the heating process are elevated.

Description

Procedure for the manufacture of objects of tool steel by powder metallurgy.

The invention relates to a method for metallurgy manufacturing of steel object powders for tools with improved homogeneity, greater purity and Better properties

The invention also refers to a tool steel object with a property profile improved.

Tool steels with high carbon concentrations and high element contents carbide formers are used for cutting parts and components with high wear resistance. As in a solidification of alloys of this type within molds of foundries form inhomogeneities as well as eutectic carbides and coarse primaries, which give rise to manufacturing problems and some bad mechanical properties of tools or components manufactured from them, it is advantageous to manufacture such parts by powder metallurgy.

A powder metallurgical manufacturing comprises fundamentally an atomization of a steel melt to tools for obtaining metal dust, an introduction and compaction of the metal powder in a container or a capsule, a tightly sealed capsule and warming and pressing Hot isostatic powder inside the capsule to get a homogeneously dense material.

When atomizing molten metal, which is advantageously carried out with nitrogen according to the current state of the technique, small gasses are formed within the gas stream metal droplets with a high surface ratio of volume, which results in a high cooling rate  and solidification of the liquid metal and, consequently, of Small particles of carbide in the dust grains. Just like mentioned above, the powder is then formed, compacted in most cases by shaking within the capsule, inside it by hot isostatic pressing at temperatures above 1080 ° C as a rule, applying a pressure of more than 85 MPa, to obtain a metallic body completely dense. This metal body a-HIPed, which can still be subjected to hot forming, it presents, with a high carbide content, a size advantageously small carbide that comes to be by term average of 1-3 um, and good mechanical properties of the material, compared to a metallurgical manufacturing by fusion.

Steel tool objects manufactured by powder metallurgy certainly show a very advantageous structure with finely distributed phases of carbides; however, and due to an incomplete isotropy of the material and a poor degree of purity, the high cannot be achieved Quality potential that can be achieved in PM materials.

The invention has been proposed to provide a remedy for this situation, and the goal of eliminating the lack of quality of steel PM objects for tools manufactured with according to the current state of the art, and present a procedure of the type mentioned at the beginning, with which you can manufacture an isostatically compacted metal body with a maximum isotropy of the material and a minimum content of inclusions oxidic

The invention also tends towards an object of tool steel with improved properties of machining and use, along with a period of duration higher.

This goal is achieved thanks to the procedure of claim 1.

The advantages obtained with the procedure to which the invention relates are due fundamentally that it improves first synergistically decisively through metallurgical work the degree of purity of oxides of a melt introduced into a container metallurgical and its temperature is adjusted homogeneously to a value of advantageous overheating, then performing a atomization of the liquid metal such that the average diameter of The grains are 50 to 70 µm. This way you get that, on the one hand, the surprisingly small oxygen content in dust and, on the other hand, also substantially increase the proportion of fine granulation with a view to achieving a high compaction by shaking and vibration within the capsule. If you proceed below, as provided in the invention, to classify the metal powder, maintaining the nitrogen atmosphere, to gather it, introduce it in a container, compact it inside it and close the container, no oxidation or oxygen fisisorption on the surface of the dust grain.

The distribution, according to the invention, of the grain diameters with an average value around 50 to 70 um allows to reach an unexpectedly high dust density inside the capsule so that, on the one hand, the shrinkage measurement during hot isostatic pressing and, on the other hand, a largely complete isotropy is achieved of the compacted dense metal body. These advantages are achieved also with containers whose dimensions have a diameter or a thickness of more than 300 mm and a length of more than 1000 mm.

The parameters for the pressing cycle or hot isostatic compacting comprise a heating of the powder inside the container with a fundamentally equal increase of temperature and pressure, so during this phase reaches, as indicated, an increase in density and homogeneity of the material. The subsequent pressing operation is performs within a temperature range of 1100ºC to 1180ºC with a pressure of 90 MPa or higher, with a duration of at least three hours, followed by a slow cooling of the compacted body Pressing temperatures below 1100ºC and pressures below 90 Mpa, as well as times Lower pressing or compacting may cause the presence of defects in the material

The compacted body shows, after HIP, a completely dense material structure, so you can machining in this state, or after heat treatment, for manufacture of a tool

For the high quality of the steel object for tools manufactured by powder metallurgy with the procedure referred to in the invention results determinant its low content of inclusions as well as the small size of them. The high oxidic degree of purity, which is documents with a K0 value according to DIN 50 602 which is fundamentally of 3 at most, not only gives rise to mechanical properties strongly improved material in all directions of solicitations, especially when elevated temperatures of utilization are high, but also greatly improves their utilization properties, preferably the hardness of the cutting edge of fine cutting tools

An especially relevant increase in quality of the object is achieved, making its manufacture according to procedure referred to in the invention, if the mass molten is formed by an iron-based alloy that contain in% of weight:

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as well as trace elements up to a total concentration of 4.8, the rest being constituted by impurities and iron. The previous chemical composition of steel for tools includes steels for especially rich tools in carbides with a high resistance to friction and a high shear resistance of tools manufactured from same. Since a high proportion of carbides worsens by general rule the mechanical properties of the material, covers a special importance its basic improvement thanks to the procedure  to which the invention refers. It has been verified that these high mechanical characteristic values, especially the material resilience, are explained synergistically by the small average diameter of the powder grains, a distribution homogeneously compact inside the capsule and the high degree of purity or cleaning of oxides with an isotropic structure of the object isostatically pressed in hot.

The degree of purity of oxides of the liquid metal It can be improved effectively by metallurgical work, provided that the melt is conditioned inside the metallurgical vessel with a turbulent current induced of the same and with a complete coating of the bath metallic by liquid slag, which will be specially heated through the direct passage of current over a period of 15 min at least An evacuation of oxygen compounds or oxides from the melt and a absorption of the same in the hot slag, increasing the efficiency the induced current of the metallic bath. Result known the technique of acting on a stream of liquid metal inside a metallurgical vessel, introducing argon gas from atomization through at least one atomization stone gas permeable located in the ground area. It is important, no However, in order to avoid a reoxidation of the mass cast, its coating is fully maintained by liquid slag, even when mass movements are made cast. To avoid problems when using a stone atomization, as regards the reliability of a configuration of an efficient and controlled metal current as well as in order to avoid difficulties in the introduction of gas from agitation and atomization, taking into account that some amounts of Small gas have little metallurgical efficiency, while some high amounts of gas leave parts of the dough surface molten without slag, which can cause oxidation as well as mixing of slag particles in the steel, preference is given to electromagnetic means, for example to stir coils electromagnetic, to create a turbulent current induced in liquid metal It can also be done about it, so very advantageous, an adjustment and a uniform distribution of the temperature of the metallic bath by means of an energy contribution thermal in the slag applied with the passage of a current electric

In another configuration of the invention, provided that the conditioned melt passes through the body of a nozzle of the metallurgical vessel, being introduced with a diameter of the melt stream from 4.0 to 10.0 mm in an atomization chamber, and being subjected thereto to the action of at least three successive jets of gas constituted by nitrogen, with a degree of nitrogen purity of 99.999% as minimum, as long as the last impact of the gas stream on the melt stream is made by a jet that present a speed that is, at least partially, greater than the Speed of sound. A maintenance of the diameter of the current of melt and the high kinetic energy of the gas action over the molten metal stream cause a distribution favorable grains and the desired fineness of metallic powder prepared. Conditioning and adjustment of the temperature of the liquid metal inside the metallurgical vessel, as well as the grade of purity of the atomization gas constituted by nitrogen explain also the surprisingly high degree of purity or the small proportion of oxygen in the powder and, consequently, in the block compacted or isostatically hot pressed.

Like even small proportions of grain thick in the metal powder, especially when filling the capsule and by compacting the powder, they can cause misdirections in it, it is advantageous to adjust or classify the diameter of the grains of powder by atomization technique at a maximum value of 500 \ mum.

In any case, it can be foreseen according to invention, in order to guarantee a homogeneous distribution and increase the quality of the product, that the powder collected in a Preparation enclosure is fluidized and mixed with nitrogen and, maintaining the nitrogen atmosphere, enter a container or in a capsule with a total weight of more than 0.5 t, compacting by shaking and closing tightly to the passage of gas.

This ensures that, if introduces the homogenized powder economically advantageously in a container or in a capsule with an equal diameter or thickness or greater than 400 mm and with a length of at least 1000 mm, Acquire homogeneity and complete density block material manufactured, applying the aforementioned parameters to the hot isostatic compaction cycle.

If the powder-filled capsule is introduced in a cold state in a HIP device and heating is performed back of the powder capsule with pressure application surrounding on all sides, you can shorten, on the one hand, the full warm-up time due to increased conduction of heat and a previous compaction can be carried out with a view to a block isotropy practically complete.

As already indicated, it may be beneficial in certain cases, in support of consolidation, the perform heating and / or powder pressing operation, applying a constant temperature, uniformly variable in its case, oscillating around an average value, and performing the operation of pressing or compaction with a temperature of 1140 ° C as minimum, and 1170ºC maximum.

Given the improved material properties it is possible, and it can be especially advantageous for minimize costs, the use of the block, manufactured with the powder metallurgy technique according to the invention, in the state as-HIPed or with a minimum conformation, performed for economic reasons, as prior material for tools or tool parts

The broad objective of achieving the invention of creating a tool steel object with some improved machining and utilization properties, along with a Longer shelf life or use, will be achieved with a tool steel object manufactured using the technique of powder metallurgy, with improved material properties which consists of an iron-based alloy that contains in %:

2

as well as trace elements up to a total concentration of 4.8, the rest being constituted by impurities and iron, said material having a K0 value of 3 at most, according to DIN 50 062

Tool steels have a broad spectrum with regard to the concentration of corresponding alloy elements that always interact to each other and to be considered regarding concentration carbon Carbon concentrations lower than 0.52% of weight results in a lower proportion of carbides and / or a lower hardness of the matrix in the thermally bonded state of steel, while concentrations exceeding 3.74% of carbon weight widely exclude the possibility, even in manufacturing case using powder metallurgy, using the tool material, because of the mechanical profile of their properties.

For good hardenability and for mechanical and chemical properties that can be achieved in the objects are especially important elements Mn and Cr, giving rise to concentrations greater than 2% of weight of Mn and greater than 21% Cr weight at a fall in the values of Material needed for the tools.

The high carbon affinity of Mo, V, Nb / Ta and W elements generate, in appropriate proportions, a Desirable configuration of carbides and mixed carbides within a alloy matrix In the sequence of the indicated elements previously, the values of concentration in weight% 10.0; 14.9; 2.0; 20.0, since in said case the desired behavior of bonus and, on the other hand, you could not reach the mechanizability and mechanical properties provided for materials.

Ni may be present in the alloy, eventually, without harmful effects with a concentration of up to 1.0% of the weight. Co increases hot hardness and resistance of the cutting edge of the tools, although it influences worsening the properties from a concentration of 20.8% Of weight. Sulfur concentrations of up to 0.5% of the weight improve machinability with steel chip removal for tools, notwithstanding the degree of purity of the same in a way that causes a reduction in values Material mechanics.

According to the invention, it presents the steel for tools a K0 value defined according to DIN 50 062 which is fundamentally 3 at most. This high degree of purity of material not only causes a great improvement of the properties mechanics in a subsidized state, for example a toughness essentially larger material, but they are also improved decisively the use or use properties, especially the resistance of the cutting edge of tools fine cut for hard objects. This increase in the quality of tool steel objects manufactured according to the invention Powder metallurgy is especially due, as has been able to verify that the low proportion of inclusions does not Small metallic and the absence of non-metallic inclusions large minimize the initiation of a cracking caused by they.

The following explains in more detail the invention, based on research results made:

From steels to work cold and fast cutting steels with some C carbon concentrations greater than 2.2% of the weight of approximately 12.5% of Cr weight and greater than 4.0 & of the weight of V, or 1.1 to 1.4% of the weight of C, of approximately 4.3% of the weight of Cr, approximately 5% of the Mo weight, 3 to 5% of the weight of V, 5.8 to 6.5% of the weight of W, up to 9% of the weight, if applicable, of Co and the rest iron and impurities, 50 pieces of loads of 8 t, in a metallurgical vessel connected to a atomization chamber, covered with a reactive slag, heating it by means of electrodes with direct passage of the stream. Within a time interval of 15 to 45 minutes melt conditioning was performed with stirring turbulent inductive of the same, always covering the surface of the melt with hot slag. Then a hole in the body of a vessel nozzle was released metallurgical, and they were injected into the stream of liquid metal that penetrated the atomization chamber with a diameter of 4.0 up to 10.0 mm successive jets of gas constituted by nitrogen, addressing the last jet of gas, which leaves the nozzle with a speed greater than that of sound, on metal liquid and breaking it down into small drops. In the chamber of atomization a solidification of the droplets in grains was performed of dust within a nitrogen atmosphere with a degree of 99.999% purity. The nitrogen atmosphere above the dust is also maintained during the process of classification and collection of same, extracting from the collecting container in each case samples for the classification of dust particles.

From the collecting vessel the powder in a container or in a non-alloy steel capsule, with a compaction of the powder filling by vibration or shaking of the container or capsule, proceeding finally at the closure of the capsule. The capsule filled with the compacted alloy powder, with a diameter of 420 mm \ diameter and a length of 2000 mm was introduced in the cold state in the HIP installation, while increasing pressure and temperature. Hot isostatic pressing was performed with a temperature of 1155ºC and a pressure of 105 MPa within a space of time of 3.85 hours, slowly cooling then the compacted body After a hot forming with a degree of conformation of 0.2 to 8.1 times a sample was taken from the forge pieces.

The 50 dust samples that were taken from container or collecting tank, using the procedure to which reference is made to the invention, they were subjected to an analysis granulometric In Table 1 (distribution or distribution granulometric metal powder) the results are reproduced, and specifically the corresponding average proportion of dust in the different kinds of particles, comparing them with 92 results obtained by applying the usual procedure according to the current state of technique

TABLE 1 Granulometric distribution of metallic powder, proportion of kinds of particles in met powder, average size of the particles

3

Powders prepared with a procedure according to the invention possessed a percentage of 52% of the total amount of particles with a grain diameter of 63 um and reached a percentage of approximately 72% up to a grain size of 100 µm. The powder manufactured according to the current state of the art on the contrary it shows in the same classes some percentages of 21.7% and 36.2%. If you compare the determined average size of the particles, it is 61 µm in case of preparation of the powder according to the invention, while in a powder manufacture carried out according to the current state of the art a average particle size twice as large with 141 \ mum.

In Figure 1 (manufacturing procedure according to the invention) and in Figure 2 (manufacturing process according to the state of the art) powders are shown in a loose distribution. In this state they occur in the dust taken to make comparisons (state of the art), as shown in Figure 2, unmixed areas with an accumulation of coarse dust grains 1 and fine fractions 2. When it comes to Powders manufactured according to the invention we find, by On the contrary, with a wide homogeneity. The same can be said of Figure 3 (preparation of the powder according to the invention) and of the Figure 4 (comparative powder) according to the state of the art.

50 blank pieces with different composition chemical in each case, manufactured according to the procedure to which referenced in the invention, tests were taken after a hot shaping and its degree of purity or the content of non-metallic inclusions according to DIN 50 062 and ASTM E 45/85 Method D. The results were again compared with results of 92 samples of materials of the same type, although manufactured according to the current state of the art, and can be seen in Table 2 (content of PM steels inclusions for tools K0) and in Table 4 (content of PM steels inclusions for tools according to ASTM value).

When conducting an evaluation of the content of inclusions in the material according to DIN 50 062, Method K0, se determined in tool steels according to the invention about Total characteristic values of up to 3, with a percentage with this value of 2%. Tool steels manufactured according to the state of the art showed by the on the contrary, as can be seen in Table 2, a content substantially higher than non-metallic inclusions, with a comparatively larger diameter. In Figure 5 you can see a graphic representation of the results of this evaluation, the characteristic values having been represented in the abscissa totals and in the ordinate their proportion in%. Accordingly curve A shows the material according to the invention and curve B a Steel manufactured according to the current state of the art.

Another concentration check was made or content of non-metallic inclusions in tool steels manufactured by powder metallurgy procedure according to ASTM E 45/85 Method D.

As shown in Table 3, determined in 50 samples of material manufactured according to the invention (Curve A), with a quantity of samples of 3 and a percentage of 6.0%, a maximum ASTM value of 1.5. With an ASTM value of 0.5 assumed the percentage 68%. The comparative material, manufactured according to the state of the art, showed a higher concentration and about coarser inclusions (Curve B), which has also shown graphic form in Figure 6, having re-represented the ASTM value on the abscissa and the proportional percentage on the neat.

Tool steels of the type indicated they can be alloyed with a sulfur concentration according to the invention, as it has been surprisingly found in the checks made, of 0.5%, but in spite of this increase substantially the content of nonmetallic inclusions and it set a K0 value according to DIN greater than 3.

TABLE 2 Content of PM steels inclusions for K0 tools (DIN 50 062)

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TABLE 3 Content of PM steels inclusions for K0 tools (ASTM E 45/85 Method D)

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Claims (14)

1. Procedure for manufacturing, by powder metallurgy, dense, deformed or not Deformed, made of high purity tool steel with a value K0 according to DIN 50 062 which is essentially a maximum of 3, in the which introduces a melt into a metallurgical vessel and it is conditioned inside it, improving the degree of purity of oxides thereof and adjusting the temperature to a value greater than the temperature of primary precipitation formation in the alloy, manufactured from said melt, with a temperature that remains fundamentally constant, a powder with an average granular diameter of 50 to 70 µm, by means of a atomization process carried out with the help of at least three successive gas jets using nitrogen with a degree of 99.999% N purity inside an atomization chamber, disintegrating it into the nitrogen stream and, keeping the nitrogen atmosphere, sorting, collecting and mixing the powder with a maximum grain diameter of 500 µm, introducing it into a container with a diameter or a thickness of more than 300 mm and a length of more than 1000 mm, compacting it inside of the same by mechanical shocks and hermetically closing the container to the passage of the gas, introducing next the container full of powder or capsule in cold state in the HIP device and adjusting the parameters in a pressure cycle hot isostatic for the container or capsule, so that the temperature and temperature increase in the heating process pressure, applied to the dust body of the container or the capsule a pressure on all sides from 1 to 40 MPa as minimum, and then performing a compression operation isostatic with a temperature of at least 1100ºC and 1180ºC at most, with an isostatic pressure of at least 90 MPa for a period of at least three hours, finally proceeding to cool the compacted HIP body and, in given case, to the subsequent hot shaping of said body compacted 2. Method according to claim 1 in the that the melt is constituted by an alloy based on iron containing in% of weight: 6 as well as trace elements up to a total concentration of 4.8, the rest being constituted by impurities and iron. 3. Method according to claim 1 or 2, in which a conditioning of the melt is carried out inside of the metallurgical vessel in a turbulent, induced current of it, preferably with the help of electromagnetic means, and with a complete coating of the metallic bath with liquid slag which is heated especially by the direct passage of a current for a period of 15 minutes as minimum. 4. Method according to claims 1 up to 3, in which the melt is introduced, through the body  of a metallurgical vessel nozzle, with a flow diameter of the melt from 4.0 to 10.0 mm in a chamber of atomization and injected into it with the help of three successive jets of gas consisting of nitrogen, taking into account that the last injection of the melt stream is done by means of a gas stream that has, at least partially, a velocity exceeding the speed of sound. 5. Procedure according to any of the claims 1 to 4, wherein the diameter of the grains of powder is adjusted or classified by means of a technique of atomization, at a maximum value of 500 µm. 6. Procedure according to any of the claims 1 to 5, wherein the powder collected in a preparation space is fluidized and mixed by means of the nitrogen and is introduced, maintaining the atmosphere of nitrogen, in a container or in a capsule with a total weight of more than 0.5 t, is compact by mechanical shocks and closes so gas tight. 7. Procedure according to any of the claims 1 to 6 wherein the powder is introduced into a container or capsule with an equal or greater diameter or thickness 400 mm and a minimum length of 1500 mm. 8. Procedure according to any of the claims 1 to 7, wherein the heating and / or the powder compression operation is performed by applying a constant temperature, which may vary regularly, oscillating around an average value, and the compression operation is performs at a temperature of 1140ºC minimum and 1170ºC at maximum. 9. Procedure according to any of the claims 1 to 8, wherein the block manufactured by powder metallurgy is used in the AS HIPed state or, with a very small conformation to be done for reasons economic, as prior material for tools or parts of tools 10. Steel tool object manufactured by powder metallurgy with material properties improved, which is preferably manufactured with a process according to the preceding claims, consisting of an alloy to iron base containing in% weight: 7 as well as trace elements up to a total concentration of 4.8, the rest being constituted by impurities and iron, said high purity material presenting a maximum value of 3 according to DIN 50 062 or an ASTM value of 1.5 as maximum according to ASTM E 45/85 Method D. 11. Object manufactured by metallurgy of powders according to claim 10, consisting of a material that presents a content of inclusions - according to the K0 procedure of DIN 50 062 - greater than 80% for the total of characteristic values 1 and 0. 12. Object manufactured by metallurgy of powders according to claim 10, consisting of a material that presents a content of inclusions - according to the K0 procedure of DIN 50 062 - greater than 50% for characteristic value 0. 13. Object manufactured by metallurgy of powders according to claim 10, consisting of a material that presents a content of inclusions - according to ASTM E 45/85 Method D - greater than 90% for the total ASTM values of 0.5 and 1. 14. Object manufactured by metallurgy of powders according to claim 10, consisting of a material that presents a content of inclusions - according to ASTM E 45/85 Method D - greater than 60% for the ASTM value of 0.5.